The present invention relates to a photosensitive composition substantially comprising a binder which is insoluble in water and soluble in aqueous-alkaline solutions, a photosensitive 1,2-quinonediazide or a photosensitive mixture comprising a compound which splits off acid upon exposure to light and a compound containing at least one acid-cleavable C--O--C group, and a substance which promotes thermal hardening. The invention also relates to a copying material which is prepared from a layer support and a photosensitive layer comprising the composition and to processes for the production of negative relief copies by exposing, heating, overall exposing, developing and optionally thermally post-hardening the copying material to increase the print run.
Photosensitive compositions, the imagewise exposed areas of which become soluble in a developer solution and the unexposed areas of which remain insoluble, have been known for a long time. For the preparation of such materials, layers containing o-quinonediazides as photosensitive compounds and which additionally contain resins with groups providing alkaline solubility, for example phenolic resins, are primarily used in practice.
Photosensitive compositions based on acid-cleavable compounds are also known. They contain orthocarboxylic acid derivatives, monomeric or polymeric acetals, enol ethers or acylimino carbonates as the acid-cleavable compounds. The radiation-sensitive compounds splitting off acid, which are contained in these compositions, predominantly comprise organic halogen compounds, in particular, s-triazines substituted by methyl halide groups. The binders used also comprise resins with groups providing alkaline solubility.
It is also known that a normally positive-working copying material based on o-quinonediazides can be used for negative processing in the presence of appropriate additives and by a particular sequence of treatments. British Patent Application No. 2 082 339 describes a reversal process of this kind, in which a photosensitive mixture comprising an o-quinonediazide and at least one resol can be employed as a negative-working recording material. German Offenlegungsschrift No. 25 29 054 (corresponding to U.S. Pat. No. 4,104,070) describes resist layers based on 1,2-quinonediazides for use in a reversal process. These layers contain a hydroxyethylimidazoline as an additive. A similar material containing secondary or tertiary amines is disclosed in U.S. Pat. No. 4,196,003. European Patent Application No. 0 133 216 (corresponding to U.S. Pat. No. 4,581,321) describes a reversal process, in which the o-quinonediazide-containing copying layer contains an additive comprising a hexamethylolmelamine ether which is preferably used as a crosslinking agent in polymer reactions, while European Patent Application No. 0 131 238 (corresponding to U.S. Pat. No. 4,576,901) discloses a reversal process, in which the photosensitive material does not require any of the above-indicated basic or acidic additives.
There has also been disclosed a positive-negative reversal process on a basis of acid-cleavable compounds instead of 1,2-quinonediazides, as described in European Patent Application No. 0 082 463 (corresponding to U.S. Pat. No. 4,506,006).
The above-described reversal processes basically comprise the same sequence of processing steps, including heating the printing plate after imagewise exposure, re-exposing the cooled plate without an original and then developing by means of an aqueous-alkaline developer.
These reversal processes are based on the fact that the photodecomposition products produced form an insoluble reaction product when they are heated. This thermal hardening preferably takes place in the presence of particular basic or acidic additives in the copying layer or in the presence of particular crosslinking agents containing multifunctional reactive groups.
Additives of this kind, however, usually have an adverse influence on the shelf life of the copying material and on particular properties relating to copying technique, for example, photosensitivity and image contrast after exposure. In addition, for many applications the temperature required for image reversal is too high, or the duration of heating at a relatively low temperature is too long, or the reversal range is too narrow.
In the reversal process described in European Patent Application No. 0 133 216, the copying layer contains, in addition to a 1,2-quinonediazide, an ether of hexamethylolmelamine as an essential constituent, as mentioned above. Although such melamine ethers advantageously promote the image reversal of a normally positive-working photosensitive material during thermal treatment in the exposed image areas, the hexamethylolmelamine ethers nevertheless have the disadvantage that the temperature tolerances of the reversal range are relatively narrow, which may give rise to varying copying results. It is also a significant disadvantage that, owing to the reactive --CH.sub.2 --O--R-- groups of the additional compound, the printing plates have a poor shelf life, in particular at elevated temperatures, which leads to premature crosslinking and thus renders the plates useless.